Apparatus of this kind is known and has often proved its worth in practice. However, it requires a considerable amount of space and, for example in the case of horizontal boring, needs a correspondingly large excavation or launch pit.
A boring apparatus for destructive replacement of an underground pipeline is known, for example, from U.S. Pat. No. 4,507,019. This apparatus comprises a frame with an adjustable mounting for two feed cylinders connected to a sliding carriage. Mounted on the carriage is a rotary drive of which the horizontal shaft is provided with an external screw thread and can be screwed on to the internal screw thread at the rearward end of a drill string made up of a number of sections. As a result of the length of the frame the known apparatus requires a large excavation or launch pit.
Added to this is the fact that when lengthening the drill string the new section of the string has to be aligned with the external thread of the drive shaft and also with the internal thread at the rearward end of the drill string or of the last section thereof, and then screwed first of all on to the external thread of the drive shaft and then into the internal thread of the drill string. This requires a double screwing operation for each section of the string and therefore is not only unnecessarily time-consuming but also increases the space required, since the distance between the drive shaft on the one hand and the rearward end of the drill string on the other hand must be greater than the length of a string section.
A further disadvantage of the known boring apparatus is that before the addition of a new section to the string the drive shaft must be unscrewed from the internal thread at the end of the drill string while the drill string is held fast by means of expensive auxiliary units to enable the screw thread of the drive shaft to be unscrewed from the end of the drill string. For this purpose German Offenlegungsschrift 41 13 422 proposes an undercarriage head provided with a clamping device and with a clamping and rotating device. The clamping device has two clamping cheeks which embrace the pipe and are driven by hydraulic cylinders in order to grip the pipe fast between them and prevent it from rotating. The clamping and rotating device includes a carrier which can rotate about the axis of the pipe on which are fitted two clamping cheeks which act on the pipe and are driven by respective cylinders. A further cylinder drives a holder so as to cause it to rotate round the axis of the pipe.
The above-mentioned disadvantages are also suffered by a boring apparatus known from German patent 41 22 350, which needs so much space that it is mounted above ground and first creates a downwardly inclined pilot bore in order to reach the laying depth and then continues boring horizontally. However, such apparatus is not suitable for laying house connections, since on account of its large external dimensions it cannot be accommodated in the sidewalk region. Furthermore it is not possible even to reach the laying depth in the sidewalk region in order to connect the house connection to a main line laid along the street.